The present invention relates to a piezoelectric ceramic transformer which can be operated by various types of high-voltage generating power supply circuits and, more particularly, to a compact, low-profile piezoelectric ceramic transformer which requires size reduction and a high reliability and generates a high voltage.
Conventionally, in a power supply circuit of a unit, e.g., a deflector of a television set or a charging unit of a copying machine, which requires a high voltage, a winding type electromagnetic transformer has been used as a transforming element for generating a high voltage. This electromagnetic transformer has a structure in which a conductive wire is wound on a magnetic core. In order to realize a high transforming ratio, the number of turns of the wound conductive wire must be increased. Therefore, it is very difficult to realize a compact, low-profile electromagnetic transformer.
Meanwhile, a piezoelectric transformer using a piezoelectric effect is proposed. FIG. 7 shows the structure of a Rosen piezoelectric transformer as a typical conventional piezoelectric transformer. This piezoelectric transformer will be described with reference to FIG. 7. Assume a case wherein a high voltage is to be obtained. In a piezoelectric plate 70 having electrodes 73 to 75 formed on its surface, a portion indicated by reference numeral 71 is a low-impedance driving section of the piezoelectric transformer. The driving electrodes 73 and 74 are formed on the upper and lower surfaces of the piezoelectric plate 70 corresponding to the driving section 71. The driving section 71 is polarized in the direction of thickness, as indicated by an arrow in FIG. 7. Similarly, a portion of the piezoelectric plate 70 indicated by reference numeral 72 is a high-impedance power generating section. The output electrode 75 is formed on the end face of the piezoelectric plate 70 corresponding to the power generating section 72. The power generating section 72 is polarized in the lengthwise direction of the piezoelectric plate 70, as indicated by an arrow in FIG. 7. This piezoelectric transformer operates as follows. When a voltage is applied to the driving electrodes 73 and 74 from external terminals 76 and 77, an electric field is applied to the driving section 71 in the polarizing direction. The piezoelectric plate 70 is vertically vibrated in the lengthwise direction in accordance with a piezoelectric effect (to be referred to as the piezoelectric transverse effect 31 mode hereinafter) wherein displacement occurs in a direction perpendicular to the polarizing direction, so that the entire transformer vibrates. In the power generating section 72, a voltage having the same frequency as that of the input voltage is derived at an external terminal 78 through the output electrode 75 in accordance with a piezoelectric effect (to be referred to as the piezoelectric longitudinal effect 33 mode hereinafter) wherein a potential difference occurs in the polarizing direction due to the mechanical strain in the polarizing direction. At this time, if the driving frequency is set to be the same as the resonant frequency of the piezoelectric transformer, a very high output voltage can be obtained. To obtain a low-voltage output upon inputting a high voltage, it is apparent that the high-impedance power generating section 72 of the longitudinal effect may be set as the input side and the low-impedance driving section 71 of the transverse effect may be set as the output side.
This piezoelectric transformer is used in the resonant state and has numerous advantages as compared to an ordinary electromagnetic transformer, such that: (1) downsizing and profile reduction can be performed since a winding structure is not needed and the energy density is high; (2) the entire structure can be made inflammable; and (3) noise due to electromagnetic induction does not occur.
However, the conventional Rosen piezoelectric transformer has drawbacks. For example, since the electrode of the power generating section is located on the loop of vibration, i.e., in the vibrating direction, connection of the external terminals adversely affects vibration or largely degrades reliability. Also, although a comparatively high output voltage can be obtained when the load resistance is extremely higher than the impedance of the piezoelectric transformer, a very high output voltage cannot be obtained when the load resistance is not much high.